1. Field of the Invention
The present invention relates to an optical disk device and a method for controlling the movement of the read/write head of the device which is applied to an optical information read/write device which functions to record, regenerate and erase information data with the use of a minute spot formed on an optical information recording medium such as an optical disk. More particularly, the present invention relates to an optical disk device and a method for controlling the head movement thereof in which the time for movement of the head is unchanged and the loop gain of the velocity data is also unchanged irrespective of change of the track pitch according to the radial position of the head on the optical disk.
2. Description of the Related Art
In accordance with an optical information read/write device such as an optical disk device, the position of the optical beam spot irradiated on the optical disk is detected in such a way that by counting the tracking error signal obtained each time when the optical beam traverses a track on the disk, the track number is measured and that the position of the optical beam, i.e., the position of the optical head is determined on the basis of the number of the tracks.
The tracking error signal is also used for controlling the velocity of the optical beam movement in such a way that the optical beam speed is detected from the tracking error signal and compared to a reference velocity data so as to control the head motion speed.
That is, the tracks formed on the optical disk are used as a scale means for detecting the position of the optical head now on being located and the velocity thereof.
Therefore, the unit of the detected velocity is arranged as (track/sec) which means the number of tracks traversed by the head in a unit of time.
In general, the unit of velocity is represented by (m/sec). That is, with respect to the process for detecting the velocity of the optical head mentioned above, the track pitch Tp (m/track) is inevitably used to obtain the head velocity.
On the other hand, to increase the memory capacity of the disk, an arrangement of a memory means is proposed wherein the track pitch is narrowed along the radial direction from the inner side of the disk toward the outer side thereof.
In accordance with such a pitch varying arrangement wherein the track pitch Tp varies as a function of the radial position thereof on the disk, the actual velocity of the optical head (represented with the use of unit m/sec) in relation to the reference velocity (unit m/sec) is controlled so that the head speed become fast in the inner side area of the disk since the track pitch Tp is large in that area, while the head speed becomes slow in the outer side periphery area of the disk since the track pitch Tp is small in that area.
On the other hand, the maximum value of the drive current for driving the optical head to move is limited for each device. Therefore, it is necessary to arrange the reference velocity value on the basis of the track pitch Tp of inner side area of the disk where a large actual velocity is required.
Accordingly, from the view of average velocity, the optical head moves at a slower speed than the speed of the head moving in the innermost side area of the disk, as a result of which the average time for moving from one track to another track becomes long in comparison to the time for the disk having a regular track pitch.
Besides, the track pitch data is input to the loop of the velocity control system, which results in that the loop gain is changed according as the track pitch Tp changes along with the movement of the optical head.
As mentioned above, in accordance with the disk device having an arrangement wherein the track pitch is narrowed from the inner side to the outer periphery side of the disk to increase the memory capacity of the disk, there are drawbacks that the average moving time of the optical head from one track to another becomes long in comparison to the disk having tracks arranged at a regular pitch since the reference velocity for controlling the actual speed of the head is determined on the basis of the track pitch Tpof the inner side area of the disk where the head has to move at a high speed and that the loop gain is varied when the pitch Tp is changed according as the optical head moves on the disk.